The present invention is applicable to microlaser with integrated optics applications, optical fiber telecommunications applications, medical applications (microsurgery, skin treatment) and semiconductor research, as well as power laser applications, these lasers emitting in the infrared spectrum (1000 to 3000 nm) or in the visible spectrum making it possible to carry out treatments of materials (welds, piercings, markings, surface treatments), photochemical reactions, controlled thermonuclear fusion or the polarization of the atoms of a gas, such as helium.
These lasers transmit on one or several discrete wavelengths with a certain degree of tuneability.
More specifically, the mixed oxides of the invention are aluminates or gallates.
Like mixed lanthanide aluminates with a magnetolead structure, there also exist lanthane-neodymium-magnesium aluminates, known as LNAs with the chemical formula La.sub.1-x Nd.sub.x MgAl.sub.11 O.sub.19 with 0&lt;x.ltoreq.1 and in particular x=0.1. These aluminates form the subject of the patents FR-A-2 448 134 and EP-A-O 043 776 and are referred to in the publication by D. Shearer and al., Journal of Quantum Electronics, vol. QE-22, No 5, 1986, p. 713-717 and entitled "LNA: a new CW Nd laser tunable around 1.05 and 1.8 .mu.m".
These mixed aluminates obtained in a monocrystalline form exhibit optical properties similar to those possessed by yttrium garnet and aluminium doped with neodymium, known under the abbreviation YAG:Nd .sup.3+, and neodymium ultraphosphate (NdP.sub.5 O.sub.14); these lasers also transmit in the infrared spectrum.
In particular, the LNA has laser emission wavelengths at 1054 and 1082 nm framing that of the YAG at 1064 nm. In addition, it has another transmission wavelength domain at about 1320 nm, a domain corresponding to the lowest attenuation by silicon optical fibers, thus allowing for transmission by optical fibers of the greatest amount of information with minimum losses.
However, the production of these aluminates in the form of monocrystals, and in particular by means of the Czochralski method most currently used in industrial applications, may result in crystals with unsatisfactory quality when industrial applications require power lasers to have large dimensions.
Moreover, the growth of these crystals is effected naturally along the crystallographic direction a. Now, the crystallographic direction c corresponding to the optical axis of the crystal is much more advantageous for laser properties and results in obtaining higher yields.
In addition, the use of a crystal whose crystallographic axis c is merged with the optical axis of the laser allows for an improved evacuation of heat and thus an improved cooling of the emitting bar when using a crystal those crystallographic axis a is merged with the optical axis; this is tied to the fact that thermic conductivity is anisotropic; it is much higher in the direction a than in the direction c.
So as to obtain an LNA bar orientated along the direction c, a Czochralski growth is then effected along the direction a, and then a sampling (or core sampling) is made of the bar obtained along the axis c. This slightly complicates the production of the laser transmitter.
In addition, the use of a bar orientated along the axis c allows for a power rise due to the improved heat removal.
The low yield of LNA laser transmission, independently of the growth problems of the latter, is due mainly to the self-extinguishing phenomenon thus limiting the quantity of the neodymium responsible for the laser effect able to be introduced into the crystal without impairing fluorescence. In the LNA, the maximum quantity of neodymium ions able to be introduced is equal to 10.sup.21 ions Nd.sup.3+ /cm.sup.3, which corresponds to x=0.25, the maximum laser intensity being obtained for x being close to 0.1.
In the LNA, the neodymium may occupy 3 crystalline sites of the structure (see the above-mentioned article by D. Shearer), which means that this neodymium exhibits defects which adversely affect heat propagation and thus the power rise of the laser. In addition, the presence of several sites for the neodymium favors self-extinction.
The partial substitution of the aluminum in the LNA by gallium with a view to improving the yield of the laser transmission by increasing the quantity of the neodymium in the structure has been described in the document FR-A-2 599 733. However, the production of the corresponding monocrystals suffers from various drawbacks (existence of bubbles, defects) due mainly to fusion non-congruence.
As another known aluminium oxide, it is possible to cite the strontium aluminate doped with neodymium having the formula SrAl.sub.12 O.sub.19 :Nd.sup.3+. This oxide is referred to in a publication by Kh. S. Bagsasarov and al. and entitled "Stimulated emission of Nd.sup.3+ ions in an SrAl.sub.12 O.sub.19 crystal at the transitions .sup.4 F.sub.3/2 --- I.sub.11/2 and F.sub.3/2 --- I.sub.11/2 in Sov. Phys. Dokl; vol. 19; No. 6, December 1974, pp. 350.
The neodymium ions in this strontium oxide may occupy several sites, thus limiting laser emission power. In addition, the crystals obtained exhibit insufficient qualities to enable them to be used in industrial laser applications and in particular in power lasers. Furthermore, the quantity of neodymium able to be introduced into this strontium oxide is extremely low, which contributes again to limiting laser power.
The document U.S. Pat. No. 4,441,049 also relates to known mixed lanthane/magnesium gallates containing strontium doped with manganese and having luminescent properties but no laser effect. These gallates are used particularly for fluorescent lighting. Furthermore, they solely exist in a powder form.
Gadolium/magnesium/strontium aluminates doped with cerium are obtained in a pulverulent form and also have luminescent properties, but no laser effect, are mentioned in the document FR-A-2 442 264.